And your post is timely… I'll note that there is indeed a typo in the GM for this Sunday's introit (Esto mihi, 6th Sunday OT): It is a Mode VI chant, not Mode IV as indicated.
As for the apparent error you noted… that's for someone else to answer!
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A typo is by definition unintentional. This is surely not a typo, but a correction (possibly the correction of a typo in the 1908!).
Go to the manuscript sources, the only neume on the syllable "-nem" is the torculus.
Given the official status of the 1908, this may be the kind of correction the editors might wish to pretend is a typo. Nevertheless, there can be no doubt that the alteration is very deliberate!
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